A photonic crystal refers to a structure in which dielectrics are regularly arranged in a manner to form a photonic band gap, thereby selectively passing or blocking an electromagnetic wave having a certain wavelength.
In recent years, many attentions have been directed to a low-loss waveguide for use in a frequency band of about 0.1 to 10 THz, which falls within a far infrared range. However, since conventional THz devices or measurement systems do not have a proper THz waveguide for the THz wave range, they transmit a THz wave into the air by using an expensive mirror or lens having, in general, a large volume. As such, several researches have been carried out to develop a low-loss THz waveguide using a metal (G. Gallot et al., J. Opt. Soc., vol. 17, p. 851, 2000), a sapphire fiber (S. P. Jamison et al., Appl. Phys. Lett., vol. 76, p. 1987, 2000) and a plastic ribbon (R. Mendis et al., J. Appl. Phys., vol. 88, p. 4449, 2000).
In the meantime, researches on a new waveguide using a photonic crystal fiber (hereinafter referred to as PCF) made of silica have been conducted throughout the world. The PCF is designed to have new characteristics that are hardly found in a conventional optical fiber. Such new characteristics include, e.g., a single mode characteristic across a wide range of frequency band (T. A. Birks et al., Opt. Lett., vol. 22, p. 961, 1997) and an air guiding characteristic exhibiting a very small transmission loss (R. F. Cregan, Science, vol. 285, p. 1537, 1999) (WO OO/37974, WO 99/64903).
However, the above-cited waveguide made of the metal, the sapphire or the plastic ribbon still exhibits a great attenuation, and, further, it is very difficult to produce a physically flexible THz waveguide by using those materials. The silica PCF also reveals the same problems when it is designed to fit the THz frequency band. Accordingly, a plastic PCF (hereinafter referred to as PPCF) capable of overcoming such drawbacks of the prior art and also improving low loss characteristic at the THz frequency band is very much in need to be developed.